An electroluminescent device (EL device) is a self-light-emitting device which has advantages in that it provides a wider viewing angle, a greater contrast ratio, and a faster response time. The first organic EL device was developed by Eastman Kodak, by using small aromatic diamine molecules and aluminum complexes as materials for forming a light-emitting layer [Appl. Phys. Lett. 51, 913, 1987].
An organic electroluminescent device (hereinafter abbreviated as an OLED) is a device changing electrical energy to light by applying electricity to an organic electroluminescent material, and generally has a structure comprising an anode, a cathode, and an organic layer between the anode and the cathode. The organic layer of an OLED, if necessary, may comprise a hole injection layer, a hole transport layer, a hole auxiliary layer, a light-emitting auxiliary layer, an electron blocking layer, a light-emitting layer (which comprises host and dopant materials), an electron buffer layer, a hole blocking layer, an electron transport layer, an electron injection layer, etc. The materials used for the organic layer may be categorized by their functions in hole injection materials, hole transport materials, hole auxiliary materials, light-emitting auxiliary materials, electron blocking materials, light-emitting materials, electron buffer materials, hole blocking materials, electron transport materials, electron injection materials, etc. In the OLED, due to an application of a voltage, holes are injected from the anode to the light-emitting layer, electrons are injected from the cathode to the light-emitting layer, and excitons of high energies are formed by a recombination of the holes and the electrons. By this energy, organic luminescent compounds reach an excited state, and light emission occurs by emitting light from energy due to returning from the excited state of the organic luminescent compounds to a ground state.
The most important factor determining luminous efficiency in an OLED is a light-emitting material. A light-emitting material must have high quantum efficiency, and high electron and hole mobility, and the formed light-emitting material layer must be uniform and stable. Light-emitting materials are categorized into blue, green, and red light-emitting materials dependent on the color of the light emission, and additionally yellow or orange light-emitting materials. In addition, light-emitting materials can also be categorized into host and dopant materials according to their functions. Recently, the development of an OLED having high efficiency and long lifespan is an urgent issue. In particular, considering EL characteristic requirements for a middle or large-sized panel of OLED, light-emitting materials showing excellent characteristics compared to conventional ones must be urgently developed. The host material, which acts as a solvent in a solid state and an energy transferer, is desirable to have high purity and an appropriate molecular weight capable for a vacuum deposition. Furthermore, the host material is desirable to have high glass transition temperature and high thermal degradation temperature to achieve thermal stability, high electro-chemical stability to achieve a long lifespan, ease of forming an amorphous thin film, good adhesion to materials of adjacent layers, and non-migration to other layers.
Also, the electron buffer layer is equipped to improve a problem of light-emitting luminance reduction which may occur due to change of current properties in the device when the device is exposed to a high temperature during a process of producing panels. Thus, the properties of compounds comprised in the electron buffer layer are important. In addition, the compound used in the electron buffer layer is desirable to perform a role of controlling an electron injection by the electron withdrawing characteristics and the electron affinity LUMO (lowest unoccupied molecular orbital) energy level, and thus may perform a role to improve the efficiency and the lifespan of the OLED.
Meanwhile, an organometallic complex having a light-emitting function such as Alq3 was conventionally used as an electron transport material in an OLED due to excellent electron transfer capability. However, Alq3 had a problem moving to another layer, and lowering color purity when used in a blue light-emitting device. Thus, a new electron transport material without the aforementioned problem and having a high electron affinity that can cause an OLED to have a high luminous efficiency due to fast electron transfer properties has been desired.
U.S. Pat. No. 8,968,887 discloses a host material comprising a phenanthrene compound as a substituent, but the host material disclosed therein must comprise a triphenylene as a backbone.
Korean Patent Application Laid-Open No. 10-2013-42901 discloses an OLED comprising a compound of a phenanthro(4,3-b)thiophene, a phenanthro(4,3-b)furan or a phenanthro(4,3-b)pyrrole as a backbone.
Korean Patent Application Laid-Open No. 10-2014-57439 discloses an OLED using a heterocyclic compound of a benzonaphtho(2,3-d)furan structure or a benzonaphtho(2,3-d)thiophene structure as a hole transport material or a host.